A solar-powered plane that developers hope to eventually pilot around the world has taken off from San Francisco Bay on the first leg of an attempt to fly across the United States with no fuel but the sun's energy.

The spindly looking plane, dubbed the Solar Impulse, departed shortly after 6 a.m. local time from Moffett Field, a joint civil-military airport near the south end of San Francisco, heading first to Phoenix on a slow-speed flight expected to take 19 hours.

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The Solar Impulse plane, piloted by Bertrand Piccard of Switzerland, takes off from Moffett Airfield in Mountain View, California as it attempts to fly across the United States

We're off! The radical plane leaves the ground on a multi-city trip across the United States It will stop for seven to 10 days at major airports in each city

The Solar Impulse is heading first to Phoenix on a slow-speed flight expected to take 19 hours

HOW IT WORKS

The aircraft runs on about the same
power as a motor scooter, propelled by energy collected from 12,000
solar cells built into the wings that simultaneously recharge batteries
with a storage capacity equivalent to a Tesla electric car.

This means the Solar Impulse can fly after dark on solar energy
generated during daylight hours, and will become the first solar-powered
aircraft capable of operating day and night without fuel to attempt a
U.S. coast-to-coast flight.

After additional stops in Dallas, St. Louis and Washington, D.C., with pauses at each destination to wait for favorable weather, the flight team hopes to conclude the plane's cross-country voyage in about two months at John F. Kennedy International Airport in New York.

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Swiss pilots and co-founders of the project, Bertrand Piccard and Andre Borschberg, will take turns flying the plane, built with a single-seat cockpit, with Piccard at the controls for the first flight to Arizona. He is scheduled to land in Phoenix at 1 a.m. local time on Saturday.

The project began in 2003 with a 10-year budget of 90 million euros ($112 million) and has involved engineers from Swiss escalator maker Schindler and research aid from Belgian chemicals group Solvay - backers who want to test new materials and technologies while also gaining brand recognition.

Project organizers say the journey is also intended to boost worldwide support for the adoption of clean-energy technologies.

With the wingspan of a jumbo jet and
weighing the same as a small car, the Solar Impulse is a test model for a
more advanced aircraft the team plans to build to circumnavigate the
globe in 2015.

The plane made its first intercontinental flight, from Spain to Morocco, last June.

The Solar Impulse pilot Bertrand Piccard, left, enters the cockpit before taking off

Pilot Bertrand Piccard gives a thumbs up before taking off in the Solar Impulse solar electric airplane at Moffett Field

The aircraft runs on about the same power as a motor scooter, propelled by energy collected from 12,000 solar cells built into the wings that simultaneously recharge batteries with a storage capacity equivalent to a Tesla electric car.

In that way, the Solar Impulse can fly after dark on solar energy generated during daylight hours, and will become the first solar-powered aircraft capable of operating day and night without fuel to attempt a U.S. coast-to-coast flight.

But the plane is unlikely to set any speed or altitude records. It can climb gradually to 28,000 feet (8,500 meters) and flies at an average pace of just 43 miles per hour (69 km per hour).

The current plane was designed for flights of up to 24 hours at a time, but the next model will have to allow for up to five days and five nights of flying by one pilot - a feat never yet accomplished.

Meditation and hypnosis were part of the training for the pilots as they prepared to fly on very little sleep.

Asked about the downside of solar-powered flight at a news conference in March to unveil the current plane, Piccard acknowledged there was a price paid for the tiny carrying capacity and massive wings.

Final preparations: Pilot Bertrand Piccard runs tests before he took off in the Solar Impulse solar electric airplane

Ground crew wait on the tarmac before the Solar Impulse plane takes off

Reporters gather around the Solar Impulse plane before it takes off

'In that sense, it is not the easiest way to fly," he said.

'But it is the most fabulous way to fly, because the more you fly, the more energy you have on board.'

He added: 'We want to inspire as many people as possible to have that same spirit: to dare, to innovate, to invent.'

The plane's four large batteries, attached to the bottom of the wings along with the plane's tiny motors, account for a quarter of its overall heft.

The aircraft's lightweight carbon fiber design and wingspan allow it to conserve energy, but also make the plane vulnerable to being tipped over.

A ground team of weather specialists, air traffic controllers and engineers track the plane's speed and battery levels and help the pilot steer clear of turbulence.

Solar Impulse cannot fly in strong wind, fog, rain or clouds. Its machinery is not even designed to withstand moisture.